1. Field of the Invention
The present invention relates to an interference reduction receiver used in a CDMA (Code Division Multiple Access) communication system.
2. Description of the Related Art
Since multipath signals that arrive at a receiver of a communication system travel through various propagation paths having different delay times due to multiple reflections, the multipath signals work as interference signals that degrade receiving quality. Various methods have been proposed in an attempt to reduce the interference.
FIG. 1 shows the outline of a conventional equalizer where sample data (a chip or an over-sampled chip) are provided to a shift register 11. The sample data are obtained by A/D (Analog Digital) converting a CDMA input signal. Values of digits of the shift register 11 are multiplied by suitable weights w1 through wn by corresponding multipliers 12, the weighted values are added by an adder 13, and an output is obtained by despreading with a predetermined spreading code by a correlation unit 14. Here, although a configuration using the shift register is illustrated in FIG. 1, the shift register may be replaced with other delaying means (the same is said of FIG. 2 and FIG. 3 that are described below).
Here, since the digits of the shift register 11 correspond to a time-axis of a delay profile 15 (impulse response) of the multipath signals, interference due to the multipath signals can be reduced by assigning suitable values to the weights w1 through wn based on a channel estimated value of a pilot signal. According to the equalizer described above, the best characteristic is obtained; however, a disadvantage is that the amount of calculation is increased because the weights have to be multiplied by the digits of the sample data.
FIG. 2 shows the outline of a conventional RAKE receiver where the sample data obtained by A/D converting the input CDMA signal are provided to the shift register 11. Despreading is carried out by the correlation unit 14 using a spreading code obtained based on a value of a digit that corresponds to a time when a peak of the delay profile 15 is detected out of the digits of the shift register 11. Suitable weights w1, w2, and so on are multiplied by the multiplier 12, the multiplied results are added by the adder 13, and an output is obtained. According to the RAKE receiver, the advantage is that the amount of the multiplying calculations is decreased, compared with the equalizer described above; however, the disadvantage is that the characteristic is slightly degraded.
FIG. 3 shows the outline of a conventional G-RAKE (Generalized RAKE) receiver (for example, Patent References 1 and 2, and Non-Patent Reference 1), where another timing is used. This timing is effective for interference removal, in addition to the timing at which the peak of the delay profile 15 is detected as performed by the RAKE receiver shown in FIG. 2. According to the G-RAKE receiver, the amount of weight multiplication is decreased, compared with the equalizer described above, and the characteristic is close to the best; therefore, the G-RAKE receiver is considered to be promising for the interference reduction.
Further, a weight “w” of the G-RAKE receiver is obtained as follows. That is, where “y” is a vector that includes output signals (complex signals) of two or more correlation units 14 as components, “z” is an output signal (a complex signal) of the adder 13, and “w” is a vector of weight, z is expressed byz=wHy
Here, H is the Hermitian transpose.
Further, where “s” is data transmitted by a specific user, “h” is a vector of a channel estimated value, and “n” is a vector of noise that includes thermal noise and multipath interference, y is expressed byy=hs+n 
In order to remove the noise component n from the output signal z, a covariance matrix R expressed by the following formula is used.R=E[nnH]
(Here, E[ ] is an expected value.)
Then, the weight w is expressed as follows.w=R−1h
FIG. 4 is a block diagram showing an example of a circuit for obtaining a component Rij of a covariance matrix for the conventional G-RAKE receiver. With reference to FIG. 4, a correlation unit 21 despreads a pilot signal (CPICH: Common PIlot CHannel) included in the received data (chip data) at a timing ti. An averaging unit 22 averages the despread signal. An adder 23 subtracts the averaged signal from the despread signal. Similarly, a correlation unit 24 despreads the pilot signal included in the received data at a timing tj. An averaging unit 25 averages the despread signal. An adder 26 subtracts the averaged signal from the despread signal. Then, a multiplier 27 multiplies the output signals of the adder 23 and the adder 26. Then, an averaging unit 28 averages the multiplied signal, and the component Rij of the covariance matrix is obtained.
Further, a technique of reducing interference for the RAKE receiver is disclosed (for example, Patent References 3 and 4), where despreading is performed at a despreading timing that is called MICT (Multipath Interference Correlative Timing). Where two path signals are considered, the MICT is a timing that is in a symmetric location of one of the two path timings with reference to the other by a delay interval between the two path signals.
[Patent reference 1] JP 2002-527927 T
[Patent reference 2] JP 2003-503879 T
[Patent reference 3] JPA 2003-133999
[Patent reference 4] JPA 2004-173793
[Non-Patent Reference 1] Gregory E. Bottomley, Tony Ottosson, Yi-Pin Eric Wang, “A Generalized RAKE Receiver for Interference Suppression”, IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 18, NO. 8, August 2000